Process for treating selenium



3,077,386 PROCES FOR TREATING SELENHUM Robert M. Blalrney, Eugene Fuerst, Mortimer Levy, and John B. Wells, all of Rochester, N.Y., assignors to Xerox Corporation, a corporation of New York No'lllrawing. Filed Jan. 2, 1958, Ser. No. 706,545 11 Claims. (Cl. 23-209} This invention relates in general to selenium and in particular to a method of improving markedly the xerographic properties of selenium for use therein.

The process of xerography was discovered by C. F. Carlson. This process is described in U.S. 2,297,691. As described therein, a base plate of relatively low electrical resistance such as metal, paper, etc.,- having a photoconductive surface thereon is electrostatically charged in the dark. The charged coating is then exposed to a light image. The charges leak off rapidly to the base plate in proportion to which any given area is exposed. After such exposure, the coating is contacted with electroscopic marking particles in the dark. These particles adhere to the area where the electrostatic charges remain forming a powder image corresponding to the electrostatic image. The powder image can then be transferred to a sheet of transfer material resulting in a positive or negative print, as the case may be, having excellent detail and quality. Alternatively, where the base plate is relatively inexpen- 'sive, as of paper or plastic, it may be desirable to fix the powder image directly to the plate itself.

From this description of the basic process of Xero .raphy it is evident that the unique properties of the photoconductive insulating layer are critical for the successful operation of the process. This layer must have a resistivity of at least about ohms-cm. in the dark in order to retain an electrostatic charge on the surface for a significant period of time and desirably it has a resistivity even greater than this. It must also have the property of lowering its resistivity several orders of magnitude on activation by radiation. The materials used by Carlson included anthracene, sulphur and various mixtures of these materials as sulphur with selenium, etc. These materials have excellent resistivity in the dark. However, they have relatively low light sensitivity. Consequently there has been an urgent need for improved photoconductive insulating materials. The discovery of the photoconductive insulating property of highly purified vitreous selenium has resulted in this material becoming the standard in commercial xerography. The photographic speed of this material is about one thousand times greater than that of the prior art photoconductive insulating materials. An exhaustive search has failed to uncover any material even approaching vitreous selenium in photographic speed when used as a vitreous coating in the xerographic process. In addition to outstanding light sensitivity, vitreous selenium is also outstanding in its dark resistivity which is at least about 10 ohms-cm.

It has been found that vitreous selenium conducts both electrons and holes but that the mobility for holes is approximately ten times that for e ectrons. Thus, vitreous selenium while possessing a long range for holes has a very short range for electrons, the minority charge carriers. So strongly does amorphous selenium trap electrons that Weimer in U.S. 2,687,484 states that the material has little or no electron conductivity. The effect of this property on the xerographic utility of selenium can best be understood by recourse now to the basic process steps of xerography. In this process sensitizing electrostatic charges are placed on the surface of the photoconductive insulating material-in this case selenium-- creating a field across the selenium (generally relative to a conductive backing). Vitreous selenium is insensitive to light having a Wave length greater than about 600 millimicrons. Shorter wavelength radiation is absorbed on or 3,ii77,385 Patented ,Feb. 12, 1963 ire closely adjacent to the free surface of the selenium. The absorption of this activating radiation acts to create hole: electron pairs in the selenium at the point of absorption. If the sensitizing charges on the selenium are negative, the positive charges created by the radiation remain at the surface to neutralize existing negative charges While the negative charges are repelled by the remaining sensitizing charges to migrate through the selenium to the conductive backing. When the sensitizing charges are positive the reverse is true-electrons created by the radiation remain at the surface to neutralize positive charges and the holes or positive charge carriers are repelled to migrate through the selenium to the conductive backing. As selenium has a very short range for electrons, when used with negative charging the result is that a large number of electrons are trapped in the bulk of the selenium thereby rendering the plate unfit for further use in Xerography until the trapped charges are freed. As selenium has a long range for holes, when used with positive sensitizing charges trapping is reduced to a sufiicieutly small degree as not to interfere with the utility of the material in the xerographic process. Accordingly, it has become the standard in xerography to use vitreous selenium films with positive polarity sensitizing charges thereon.

There are many applications of xerography wherein the short range for the minority charge carriers constitutes a critical deficiency. Thus, in xeroradiography, X-rays penetrate throughout the entire film of selenium generating hole-electron pairs throughout the layer rather than merely on the surface as in the case of the normal xerographic process. Furthermore, in order to absorb as large a portion of X-ray energy as possible, xeroradiography requires the use of selenium films considerably thicker than those used in normal xerography. A normal xerographic plate, that is, one designed for used with visible radiation, is generally from 20 to 50 microns thick. In xeroradiography the selenium films are from to microns thick. The result of this combination of circumstances is that to be useful in xeroradiography the photoconductive insulating material must have an appreciable range for both holes and electrons.

Other applications also exist Where it is essential that the photoconductive insulator have a long range for both polarities of electrostatic charge carriers. Thus, in U.S. patent application Ser. No. 437,460, filed June 17, 1954, by John Bardeen, now abandoned, there is described a novel plate structure whereby the response of a xerographic plate to visible radiation is multiplied or enhanced leading to photographic emciency several times that obtained with the same materials in conventional plate structures. One of the preferred plate structures described by Dr. Bardeen uses vitreous selenium as the photoconductive insulating material. To be useful in the described plate structure, it is essential that the selenium have a long range for both holes and electrons.

Still another area wherein it is desirable that the vitreous selenium have a long range for both polarities of charge carriers is in obtaining a reversal of the image to be reproduced in the normal xerographic process. In this case if th normal xerographic plate is charged negatively and then the steps of the xerographic process carried through including development with carriers and toners as prescribed for the normal xerographic process, there is obtained a negative or reversal image of the copy being reproduced. Thus, if the plate has a long range for both polarities of charge carriers it is possible merely by altering the polarity of the sensitizing charges to obtain either a positive or reversal reproduction of the subject material being reproduced.

There has now been discovered a process for treating selenium whereby the material acquires the unique property of having a long range for both polarities of charge carriers. The mechanism of trapping the electrons in vitreous selenium is still not understood but it is believed to be somewhat as follows: Vitreous selenium forms long chains with an unpaired electron at each end. This unpaired electron creates a possible electron trap. One possibility here, for instance, is that if two or more electron chains terminate in the same area there is an increased possibility of an electron trap. it is also known that certain structures of oxygen and selenium are isomorphous. If one of these selenium chains terminates in oxygen the trap is likely to be deeper due to the more electronegative character of oxygen. Even a few parts of oxygen per million parts of selenium, an amount well below the threshold of ordinary analytical detection, would be sufiicient to provide a high density of electron traps. Acting on this reasoning applicants have found that if selenium in the fluid state is intimately contacted with an oxygen acceptor and, in particular one possessing two free electrons, the material acts to remove oxygen and may also supply the need of the unpaired electron in the selenium chain thus filling the trap. It is also essential that the conditions of evaporation for the preparation of the selenium plate be carried out so that further contamination of the selenium with oxygen does not occur.

It has been found that selenium treated in accordance with the theory herein described has a significantly longer range for electrons. Material so treated is characterized by truly outstandingly low dark decay and light fatigue (two of the most significant consequences of having a short range for the minority charge carriers are a high dark decay and light fatigue). Xerographic plates prepared with selenium treated in accordance with the instant invention may be sensitized with either polarity of electrostatic charges for use in the xerographic process.

The general nature of the invention having been set forth the following examples are now presented as illustrations but not limitations of the methods and means of carrying out the invention. In the following examples all the plates were prepared by vacuum evaporation at a pressure from about 5x10 to 3x10 millimeters of mercury. In each case the conductive backing member consisted of an aluminum plate anodized so as to have a thin, dense coating of aluminum oxide thereon. The backing plate was mounted on a platen during evaporation. No attempt was made to control the temperature of the backing plate. The evaporator boat used consisted of Alundum, a trademark of the Norton Co. for a fused alumina refractory. The temperature of the boat during evaporation was maintained at 250300 C. The dark decay and light fatigue of the plates in the following examples were determined as follows: One part of the plate was exposed for two minutes to a 100 watt incandescent bulb held one meter away. The light was turned off and the entire plate charged in the dark with a corona charging unit. The charge on the portion of the plate never exposed to light was then measured (all plate charges were measured with a D.C. electrometer). Three minutes later the charge on the same portion of the plate was remeasured and, in addition, the charge on the area which had been exposed to light prior to charging was measured. To illustrate the calculation of dark decay and light fatigue, if the initial charge was 600 volts, the charge in the dark area three minutes later was 500 volts, and the charge in the previously exposed area three minutes later was 450 volts, the dark decay would be or 16% and the light fatigue would be Example 1 A lot of selenium was divided into two portions. One portion, serving as a control, was used to prepare a xerographic plate as described above. The second portion was used to prepare a xerographic plate under identical conditions excepting that 10 grams of chips of a 316 stainless steel alloy were placed in the open Alundum boat and 75 grams of selenium pellets placed on top. T e xerographic plates so prepared each had vitreous selenium layers 160 microns thick. The control plate had a dark decay of 35% and a light fatigue of 16%. The selenium treated with the stainless steel chips had only 13% dark decay and 12% light fatigue.

Example 2 One lot of selenium was divided into six port-ions and separate xerographic plates made from each portion as described above. In each case the selenium layer was 160 microns thick. The first plate served as the control. In the preparation of the other plates the following amounts of 4-0 mesh iron powder were sprinkled on top of 75 gms. of selenium pellets in the Alundum boat: 5 gms., 10 gms., 18 gms., 35 gms., and 50 gms. Evaporation conditions were otherwise identical. The control plate had a dark decay of 52% and a light fatigu of 70%. The selenium treated with 5 grams of iron had a dark decay of 3% and a light fatigue of 4%. The plate treated with 10 grams of iron had a dark decay of 2% and a light fatigue of 2% and the plates treated with 18, 35 and 50 grams each of iron all had dark decays of 0.0% and only 1% light fatigue.

Example 3 Two xerographic plates were prepared as described above from separate portions of the same lot of selenium. The resulting plates had layers of vitreous selenium microns thick. The evaporation conditions were identical except that in the preparation of one plat 3% by weight of chromium was sprinkled on top of the selenium pellets in the Alundum boat. The control plate had a dark decay of 32% and a light fatigue of 32%. The plate prepared with the chromium treated selenium had a dark decay of only 1% and no detectable light fatigue.

Example 4 Two xerographic plates were prepared as above from separate portions of the same lot of selenium. The evaporation conditions were identical except that in the preparation of one of the plates 8 grams of granular ferrous sulfide were sprinkled on top of the 75 gram charge of selenium pellets in the Alundum boat. The plates so prepared each had a layer of vitreous selenium microns thick. The control plate had a dark decay of 20% and a light fatigue of 84%. The plate prepared with the ferrous sulfide treated selenium had a dark decay of 3% and a light fatigue of 3%.

Example 5 A lot of selenium was divided into two portions. One portion was used for a control and the other was placed in a glass ampule and an amount of titanium wire equal to 7%% of the weight of the selenium was placed at one end of the ampule. The ampule was evacuated to 20 to 30 microns of mercury pressure, sealed and then heated in an oven at 525 F. for two hours after the selenium had melted with constant contact between the molten selenium and the titanium wire. At the end of the treatment the ampule was tipped so that the molten selenium drained away from the wire and then cooled. The ampule was broken and the selenium broken up.

Two xerographic plates were then prepared as described above, each having a layer of vitreous selenium 80 microns thick. The control plate (prepared from the untreated selenium) had a dark decay of 69% and a light fatigue of 34%. The plate prepared using the a light fatigue of 43%.

of 2%"and a light fatigue of titanium treated selenium had a dark decay of only 4% and a light fatigue of 6%.

Example 6 A lot of selenium was divided into 5 portions. One portion was used for preparing an 80 micron control plate (all plates in Example 6 had an 80 micron layer of vitreous selenium). The second portion was placed in a glass ampule and treated with 7 /2% by weight (based on the selenium) of titanium wire as described above. The third batch was also treated in a glass ampule using the same titanium wire used in treating the previous hatch. The fourth lot of selenium was treated in a glass ampule using the same titanium wire used to treat the second and third lots. The fifth lot of selenium was treated in a glass ampule as described above using the same wire used in treating the second, third and fourth lots. Prior to using the wire for the fifth lot, however, it was given a deoxidizing treatment by'first dipping in potassium cyanide and then etching with acid. The control plate had a dark decay of 30% and a light fatigue of 95%. The plate prepared from the selenitun treated for the first time with the titanium Wire had adark decay of 1% and a light fatigue of 4%. The plate prepared from selenium treated with the once used titanium had a dark decay of 1% and a light fatigue of 7%. The plate prepared from the selenium treated with thetwice used titanium wire had a dark decay of 7% and The plate prepared from the selenium treated with the cleaned titanium wire had a dark decay of 6% and a light fatigue of 5%.

Example 7 larly treated with aluminum wire based on the Weight of the selenium. A xerographic plate 160 microns thick was prepared as above from each lot of selenium. The control plate had a dark decay of 45% and a light fatigue of 38%. The plate prepared from selenium treated with the 2 /2% aluminum wire had a dark decay The plate prepared from the selenium treated with 10% by weight of aluminum wire had a dark decay of 2 /2% and a light fatigue of 10%.

Example 8 To illustrate the detrimental effect of minor amounts of oxygen, four plates were prepared from the same lot of selenium. The selenium used was the type described above having a long range for holes and electrons. A

first lot of this selenium was used as the control. The

second lot was heated in air for one hour at 240 to 260 C. For the third lot, one-half gram of powdered selenium dioxide was sprinkled on top of 75 grams of the selenium in the Alundum boat. The fourth lot was treated as will he described hereafter. The control, that is, the xerographic plate prepared from the selenium having a long range for both holes and electrons, had a dark decay of 5% and a light fatigue of The plate prepared from the selenium which had been heated in air had a dark decay of 34% and a light fatigue of 60%. The plate prepared from the selenium treated with selenium dioxide had a dark decay of 21% and a light fatigue of 69%. The fourth lot of selenium was evaporated without special treatment excepting that the vacuum evaporator was flushed with oxygen for 10 minutes prior to the beginning of evaporation. Throughout the evaporation the pressure in the evaporator was kept at 0.5 to 0.7x 10- millimeters of mercury. Oxygen was bled into the evaporator throughout evaporation, the rate being adjusted so that the vapor pressure did not exceed the limitations stated. The plate so prepared had a dark decay of only 7% but a light fatigue of 49%.

Example 9 A xerog'raphic plate was prepared as above excepting that the Alundum boat was completely wrapped in a mesh Monel metal screen so that the selenium vapor had to pass through the screen to reach the aluminum hacking member. The boat was heated by passing electrical current through the Monel screen. The screen restricted the evaporation rate so that twice the usual time was required to complete the coating operation. The xerographic plate so prepared was characterized by having a long range for both charge carriers as evidenced by an absence of light fatigue.

If the selenium treated in accordance with the instant invention is to be used in the preparation of xerographic plates, great care must be taken not to recontarninate the selenium with oxygen. Thus, as shown in Example 8, if the xerographic plate is prepared by vacuum evaporation, the oxygen concentration must be kept at less than about 0.7x 10* millimeters of mercury. Similarly, if the xerographic member is prepared by means other than vacuum evaporation, as for example, melting and pressing or spraying molten selenium onto the base member, or, if desired, by flame deposition, chemical deposition, etc., the operation should be carried out in an inert atmosphere so that recontamination does not occur.

A wide variety of means for contacting the selenium with the oxygen acceptor material is available. So long as intimate contact does occur, the process is not critical. Thus, the selenium may be contacted with the treating material before evaporation (as in the wire treatment), during evaporation (as by adding the material to the selenium in the boat, using the material to form the boat itself, etc), or again, the contact may occur with the selenium vapor (as in Example 9 when the vapor passed through a screen of the treating material). To assure intimate and thorough contact it appears essential that the selenium be in fluid form, either liquid. or vapor.

The oxygen acceptor material itself must be a metal or a compound of a metal which has a positive electrode potential in the standard Electromotive Force Series. Where the compound of a metal is used, there must be present a lower valence form of the metal in the com pound. It is desirable, although not essential, that the metal have two free electrons. Thus, chromium, nickel, iron, zinc, calcium, titanium, and similar materials serve as efiicient treating materials. Aluminum, having three free electrons but ranking high in the standard Electromotive Series, is also highly effective. On the other hand, copper and silver, both having negative electrodepoteutials in the Electrornotive Series and having only one free electron have been found to be inoperative.

The quantity of oxygen acceptor material per amount of selenium treated is not critical. 'lhere must be sufficient amount to intimately contact all of the selenium under the conditions of treatment. Thus, the minimum amount is, to some extent, dependent on the method of treatment. Although the amount of oxygen in xerographic-quality selenium is very slight, the oxygen acceptor material is apparently deactivated or poisoned in the process quite out of proportion to the amount of oxygen in the selenium. While very large amounts of selenium. The instant process achieves its noveland use aorzeee 7 ful result by eliminating traps, not by supplying carriers for electrons. (Achieving equality in the range of both holes and electrons by supplying additional carriers for electrons would be detrimental to the insulating properties of the selenium thus rendering it unfit for use in the xerographic process.)

Selenium treated in accordance with the instant invention may be used wherever a photoconductive insulating material is desired as in television camera tubes, etc. The material is particularly useful in xerography in preparing xerographic plates wherein it is often essential that the photoconductive insulator possess the property of having a long range for both polarities of charge carriers as shown above.

When used in preparing xerographic plates, the selenium treated in accordance with the instant invention may be used as the sole photoconductive insulating material or may be alloyed with modifying elements as tellurium, arsenic, arsenic trisulfide, etc., as shown by example in U.S. 2,803,541, US. 2,803,542, and U.S. 2,745, 327.

Xerographic quality selenium is commercially available from commercial selenium suppliers. This grade is the highest purity selenum available on the market and generally specifies that no more than about 20 parts impurity per million parts of selenum may be present. Larger impurity concentrations obscure the electrical properties of the selenium itself unfitting it for use as a photoconductive insulator as is well known to those skilled in the art. When used in a xerographic plate, the thickness of the photoconductive insulating layer is between about 20 and 200 microns. The backing member therefor is selected according to conventional requirements for the xerographic art and generally comprises a metallic plate, cylinder, sheet, web or the like or other backing material having structural characteristics and being electrically conductive either by its inherent nature or by having an electrically conductive material dispersed throughout its volume or coated thereover. Suitably this backing member may be a metallic member such as a member of aluminum, brass, magnesium, iron, steel, chromium, zinc, or the like or may be of another electrically conductive material such as electrically conductive glass, metallized paper or plastic or the like or electrically conductive resins, plastic or like materials.

While the preparation of xerographic plates has been described under conditions of high vacuum, it is understood that the conditions of evaporation may include evaporation at atmospheric pressure so long as the evaporation is carried out in an inert atmosphere such as helium, argon, etc., with oxygen carefully excluded.

It is, of course, understood that the instant invention relates to improving the characteristics of vitreous (i.e., amorphous) selenium and not the other allotropic forms thereof.

While the instant invention has been described principally in terms of removing oxygen, it is to be understood that the amounts of oxygen in xerographic quality selenium both before and after treatment as described herein are so low as to be undetectable by chemical or even spectroscopic analysis. It is known, and as fully shown in the examples herein, that materials which are oxygen acceptors when used to treat selenium in accordance with the instant invention are enormously effective in improving the xerographic qualities thereof; and, conversely, that exposure to elemental oxygen causes a pronounced loss in desirable xerographic properties.

We claim:

1. A process of treating substantially pure selenium to reduce the light fatigue and dark decay of xerographic plates prepared therefrom comprising:

contacting said selenium while heated at least above the melting point thereof and in the absence of oxygen with at least about 2% of additional material 3 and separating said additional material from said selenium,

said additional material being stable and non-volatile at least at the melting point of selenium,

said additional material being adapted to react with oxygen to form a product non-volatile at least at the melting point of selenium,

said additional material being selected from the group consisting of iron, chromium, ferrous sulfide, titanium, aluminum, nickel, and alloys and mixtures thereof,

said additional material having a surface substantially free from prior contact with selenium,

said additional material being present in an amount sufiicient to reduce the light fatigue and dark decay of a xerographic plate prepared from said selenium. 2. The method of claim 1 in which said material comprises titanium.

3. The method of claim 1 in which said material comprises iron present in an amount of at least 12% based on the weight of the selenium.

4. The method of claim 1 in which said material comprises chromium.

5. The method of claim 1 in which said material comprises aluminum.

6. The method of claim 1 in which said material comprises ferrous sulphide.

7. A process of making selenium layers onto a member, said layers having reduced light fatigue and dark decay when employed in xerography, comprising:

evaporating substantially pure selenium onto said member from a boat in an atmosphere containing less than about O.7 l0- rnm. oxygen, and adding to the selenium in the boat and before evaporation at least 2% of additional material, said additional material being stable and non-volatile at least at the evaporation temperature of selenium,

said additional material being adapted to react with oxygen to form a product non-volatile at least at the evaporation temperature of selenium,

said additional material being selected from the group consisting of iron, chromium, ferrous sulfide, titanium, aluminum, nickel and alloys and mixtures thereof,

said additional material having a surface substantially free from prior contact with selenium,

said additional material being present in an amount sufiicient to reduce the light fatigue and dark decay of a xerographic plate prepared from said selenium.

8. The method of claim 7 in which said material Comprises titanium.

9. The method of claim 7 in which said material comprises at least 12% iron based on the weight of selenium.

10. The method of claim 7 in which said material c mprises chromium.

11. The method of claim 7 in which said material comprises aluminum.

References Cited in the file of this patent UNlT ED STATES PATENTS 2,121,603 Lotz June 21, 1938 2,307,474 Thompson Ian. 5, 1943 2,409,835 Clark Oct. 22, 1946 2,414,295 Gardner Jan. 14, 1947 2,462,949 Boer Mar. 1, 1949 2,474,966 Addink et al. July 5, 1949 2,567,251 Stitt Sept. 11, 1951 2,656,664 Hadley July 14, 1953 2,662,832 Middleton Dec. 15, 1953 2,663,636 Middleton Dec. 22, 1953 2,736,672 Klein Feb. 28, 1956 2,753,278 Bixby July 3, 1956 (tithes references on following page) 9 UNITED STATES PATENTS Paris Aug. 20, 1957 Vyverberg Oct. 8, 1957 Von Stein Dec. 10, 1957 Vaaler May 20, 1958 5 Bueker et a1. NOV. 18, 1958 10 Allison Dec. 1, 1959 Oberbacher Mar. 29, 1960 Bixby Feb. 7, 1961 FOREIGN PATENTS France Mar. 26, 1952 Germany July 31, 1958 

1. A PROCESS OF TREATING SUBSTANTIALLY PURE SELENIUM TO REDUCE THE LIGHT FATIGUE AND DARK DECAY OF XEROGRAPHIC PLATES PREPARED THEREFROM COMPRISING: CONTACTING SAID SELENIUM WHILE HEATED AT LEAST ABOVE THE MELTING POINT THEREOF AND IN THE ABSENCE OF OXYGEN WITH AT LEAST ABOUT 2% OF ADDITIONAL MATERIAL AND SEPARATING SAID ADDITIONAL MATERIAL FROM SAID SELENIUM, SAID ADDITIONAL MATERIAL BEING STABLED AND NON-VOLATILE AT LEAST AT THE MELTING POINT OF SELENIUM, SAID ADDITIONAL MATERIAL BEING ADAPTED TO REACT WITH OXYGEN TO FORM A PRODUCT NON-VOLATILE AT LEAST AT THE MELTING POINT OF SELENIUM, SAID ADDITIONAL MATERIAL BEING SELECTED FROM THE GROUP CONSISTING OF IRON, CHROMIUM FERROUS SULFIDE, TITANIUM, ALUMINUM, NICKEL, AND ALLOYS AND MIXTURES THEREOF, SAID ADDITIONAL MATERIAL HAVING A SURFACE SUBSTANTIALLY FREE FROM PRIOR CONTACT WITH SELENIUM, SAID ADDITIONAL MATERIAL BEING PRESENT IN AN AMOUNT SUFFICIENT TO REDUCE THE LIGHT FATIGUE AND DARK DECAY OF A XEROGRAPHIC PLATE PREPARED FROM SAID SELENIUM 